Avian

Standardization of slaughter by live weight in Caipira slow-growing broilers using nonlinear mixed models and genetic parameters for growth and carcass traits

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Abstract

Flock uniformity and carcass standardization are major challenges in poultry; achieving homogeneity facilitates management and increases overall efficiency. In commercial systems, birds are typically slaughtered at fixed age under an all-in, all-out scheme, which results in substantial variability, particularly in populations with less intensive selection. This study evaluated target-weight slaughter (2.6 kg) and early growth-based stratification. Weekly body weight and carcass data were collected from 74 meat-type (MT; lines Pinto Preto Pesado de Pasto, Pescoço Pelado de Piracicaba, and ESALQ Caipirào) and 74 dual-purpose (DP; lines ESALQ Caipirinha and Carijó Barbada) broilers from 7 to 91 days of age. Birds were slaughtered weekly upon reaching the target weight. Growth trajectories were modeled using a nonlinear mixed-effects Gompertz-Laird model (nlme package, R), with animal-level random effects on the A, b, and c parameters, assuming a multivariate normal distribution with a fully parameterized covariance matrix. Fitted curves were differentiated to estimate the age at the 2.6 kg target weight. The model was applied to an expanded dataset of 3,010 birds, and parameters were incorporated into a Bayesian multivariate genetic model (MCMC) with carcass traits, based on a pedigree of 3,897 individuals (GIBBSF90+). Target-weight slaughter significantly reduced variability between categories and ensured high carcass uniformity, addressing the inherent limitations of fixed-age slaughter. When adjusted slaughter ages were analyzed for similar growth trajectories (e.g., 78 days), MT birds reached the target weight at 73 days, while DP birds reached it at 76 days. The interval between peak weight gain and adjusted slaughter age was significantly shorter for MT (22±7 days) than for DP birds (46±22 days), with the latter also exhibiting much greater slaughter age variability (74 to 152 days), likely associated with simultaneous genetic selection for egg production. These results demonstrate that weight-based slaughter is efficient for growth and carcass standardization, allowing birds from heterogeneous lines to reach market weight at similar ages within sub-flocks grouped by growth performance, whereas fixed-age slaughter fails to account for such intra-flock variability. Regarding carcass traits, weight-based standardization effectively mitigated differences in most cuts (p > 0.05). Estimated heritabilities for growth parameters (A = 0.44; b = 0.59; c = 0.69) confirm that growth dynamics are genetically determined, with moderate to moderate-high correlations with most carcass traits. Forty-two days of age emerged as a strategic point for group separation, pending future RRM analysis validation. This milestone enables the practical allocation of birds into distinct rearing groups, facilitated either by trained personnel or automated weight-based imaging and AI systems, to implement specialized nutritional, environmental, and slaughter protocols for each growth profile. By customizing management for divergent trajectories, this approach optimizes space utilization and biosecurity, establishing weight-based stratification as a robust strategy for managing heterogeneous poultry populations.

Keywords: 2026

How to Cite:

Balieiro, J., Dilelis de Rezende Souza, F., Franzo, V., Mazzero, L., Mourà£o, G., Paiva, L. & Ventura, R., (2026) “Standardization of slaughter by live weight in Caipira slow-growing broilers using nonlinear mixed models and genetic parameters for growth and carcass traits”, World Congress on Genetics Applied to Livestock Production Digital Archive 2026(1): 2287382. doi: https://doi.org/10.31274/wcgalp.24285

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